The Child's Book of Nature / Three parts in one

by Worthington Hooker

I will explain this to you a little more particularly. When you put the cork into the end of the gun, the barrel is full of air. Now, if the cork were not in, as you pushed the rod the air would all go out before it; but the cork in the end keeps all the air in. As you push the rod, you crowd the air into a smaller space. If you push the rod half way, then the same air that filled the whole gun has half the room that it had before you pushed the rod. Now, when air is pressed or crowded in this way, it tries, as we may say, to get away from the pressure. In doing this, it presses on the cork; but the cork sticks fast in the mouth of the gun till the pressure is enough to push it out, and when it gives way it does it all at once, and so makes the popping sound. It is as if the air gave the cork a sudden kick, and out it flies.

Explanation of the potato and quill pop-gun.

When I was a boy, we had no such nice pop-guns as boys now have. We had to make them ourselves. We would sometimes make the tube or barrel part out of elder, which, you know, has a large pith. We would sometimes take a quill for a barrel. To this we would fit a stick as a rod. We would then punch each end of the quill through a thin slice of raw potato. This would, of course, leave a round piece of potato in each end. Now, by pushing the rod quickly through the quill, the piece of potato in the farther end would fly out with a pop, in the same way that a cork does from the pop-guns nowadays. You see how this is done. The air which is shut up in the quill between the two pieces of potato is crowded into a small space when the stick is pushed in. It tries to escape from this pressure, and so presses on the potato at the farther end. This gives way all at once and flies out. But why must we have the potato in both ends? It would not be necessary if the stick could be made to fit the quill exactly; but it can not, and so there would be a leaking of air by it if we should have the potato in only one end. The piece of potato in the end where you put in the stick prevents this leaking of air. It makes, in fact, a tight piston for the stick to work.

It is the springiness of the air that makes the pop-gun work. This you can see by some experiments. Fill your pop-gun with water, and see how different from the air it will act. The moment that you push the rod, the cork will be pushed out without any popping, and the water will run out. What is the reason of this? It is because you can not crowd the water, as you do the air, into any smaller room. It moves straight along, and pushes out the cork.

Experiments with the pop-gun.

As the water can not be crowded into any smaller space, it has no spring. But the air can be shrunk up, as we may say, by pressure, and it is ready to swell out again whenever it can have a fair chance to do so; and the harder you press it, the greater is this springiness. You can see that this is true by a little experiment that you can try with your pop-gun. Press the cork end of the gun firmly against something, so that the cork can not come out. Now push in the rod quickly, and then let go of it. It will fly back, because the crowded air, by a spring, throws it back. And the harder you push it in, the more forcibly will it fly back.

Now, if you try the same experiment with the water in the gun, you will find that you can not push the rod unless the gun leaks, and then the water will come back by the piston. Why is this? It is because the water can not be crowded into any smaller space, as the air can be. If it could be, the water would do just as well in the pop-gun as air does.

You see, then, that it is the spring of the air that forces the cork out of the gun; and the air has this spring because it is pent up and crowded together, as we may say, into so small a space. It wants more room, and pushes to get it.

The cork is shot out of the pop-gun in the same way that the ball is shot out from the cannon. The air, pent up in a little space in the pop-gun, does the same thing as the gas, pent up in the cannon, does. The air wants more room, and so it kicks out the cork; and the gas, so suddenly made out of the powder, wants more room, and so it kicks out the ball. The gas has the same springiness that the air has.